1. Field of the Invention
The present invention relates to an optoelectronic integrated circuit. Particularly, the present invention relates to an optical waveguide device having functions to perform polarization rotation.
2. Related Background Art
There is known an optical waveguide device having a function to output the inputted light by rotating its polarization plane, referring to one of the opto-electronic integrated circuits (OEIC), disclosed, for example, in Rod. C. Alferness, "Electrooptic Guided-Wave Device for General Polarization Transformations", IEEE Journal of Quantum Elextronics, Vol. QE-17, No. 6, June 1981, pp. 965-969.
For this known optical waveguide device, a mode converting element of phase compensation type is configured by providing a phase shifter and a mode converter along a waveguide formed on its substrate. The phase shifter has a function to shift phases relatively between TE and TM components (being at a right angle to each other) of incident light, while the mode converter has a function to perform the mode conversion of the TE and TM components. In order to rotate the polarization plane with this phase compensation type mode converting element, it is required to provide the TE and TM components with a phase difference of 90.degree. at the inlet of the mode converter. Hence, the above-mentioned phase shifter is arranged before the mode converter to make the adjustment of the phase difference possible.
However, in a conventional optical waveguide device such as this, there has been a problem encountered that although the device functions effectively for light of a single wavelength, a desired rotation of the polarization plane is not obtainable when the wavelength spectra vary. In particular, while passing through the waveguide path, the TE and TM components produce a phase shift, but the amplitude of this phase shift varies for each wavelength. Accordingly, the phase difference at the inlet of the mode converter changes with the wavelength. As a result, a wavelength dispersion is generated for the rotation of polarization plane of the output light, making it impossible to obtain a desired rotation of the polarization plane at any wavelengths other than a specific wavelength.
For example, therefore, when the wavelength spectra range of light is broad in passing through the waveguide path as in the case of a superimposition of RF (radio frequency) on a laser diode of a light source for the reproducing head of magneto-optical recording, it is impossible to employ the conventional optical waveguide device.